A newborn case with carnitine palmitoyltransferase II deficiency initially judged as unaffected by acylcarnitine analysis soon after birth

Logistic role of carnitine shuttle system on radiation-induced L-carnitine and acylcarnitines alteration

PURPOSE

With the development of radiation metabolomics, a large number of radiation-related metabolic biomarkers have been identified and validated. The L-carnitine and acylcarnitines have the potential to be the new promising candidate indicators of radiation exposure. This review summarizes the effect of carnitine shuttle system on the profile of acylcarnitines and correlates the radiation effects on upstream regulators of carnitine shuttle system with the change characteristics of L-carnitine and acylcarnitines after irradiation across different animal models as well as a few humans.

CONCLUSIONS

Studies report that acylcarnitines were ubiquitously elevated after irradiation, especially the free L-carnitine and short-chain acylcarnitines (C2-C5). However, the molecular mechanism underlying acylcarnitine alterations after irradiation is not fully investigated, and further studies are needed to explore the biological effect and its mechanism. The activity of the carnitine shuttle system plays a key role in the alteration of L-carnitine and acylcarnitines, and the upstream regulators of the system are known to be affected by irradiation. These evidences indicate that that there is a logistic role of carnitine shuttle system on radiation-induced L-carnitine and acylcarnitines alteration.

Need for strict clinical management of patients with carnitine palmitoyltransferase II deficiency: Experience with two cases detected by expanded newborn screening

In Japan, carnitine palmitoyltransferase II (CPTII) deficiency has been included as one of the primary target diseases in the expanded newborn mass screening program since 2018. However, many cases of the severe infantile hepatocardiomuscular form of CPTII deficiency showed severe neurodevelopmental delay or sudden death, which indicated that management of CPTII deficiency in the acute phase remains to be studied in detail. Herein, we discuss two cases diagnosed by newborn mass screening. Patient 1 was under strict clinical management from the neonatal period, with >20 admissions in 14 months, while Patient 2 was managed using a relatively relaxed approach, with only 2 admissions in the same period. Patient 1 showed normal development; however, Patient 2 expired at the age of 1 year 2 months. To develop strategies for preventing sudden deaths in patients with CPTII deficiency, this retrospective study focused on detailed clinical management practices and biochemical findings during the acute phase. We also investigated the correlation between conventional biomarkers (such as creatine kinase) and long-chain acylcarnitines. We propose that strict monitoring and immediate medical attention, even in case of slight fever or minor abdominal symptoms, can help prevent sudden death in patients with CPTII deficiency. Considering the higher morbidity rate of such patients, strict and acute management of CPTII deficiency cannot be overemphasized.

Metabolic interactions between peroxisomes and mitochondria with a special focus on acylcarnitine metabolism

Carnitine plays an essential role in mitochondrial fatty acid β-oxidation as a part of a cycle that transfers long-chain fatty acids across the mitochondrial membrane and involves two carnitine palmitoyltransferases (CPT1 and CPT2). Two distinct carnitine acyltransferases, carnitine octanoyltransferase (COT) and carnitine acetyltransferase (CAT), are peroxisomal enzymes, which indicates that carnitine is not only important for mitochondrial, but also for peroxisomal metabolism. It has been demonstrated that after peroxisomal metabolism, specific intermediates can be exported as acylcarnitines for subsequent and final mitochondrial metabolism. There is also evidence that peroxisomes are able to degrade fatty acids that are typically handled by mitochondria possibly after transport as acylcarnitines. Here we review the biochemistry and physiological functions of metabolite exchange between peroxisomes and mitochondria with a special focus on acylcarnitines.

Evaluation of Metabolic Defects in Fatty Acid Oxidation Using Peripheral Blood Mononuclear Cells Loaded with Deuterium-Labeled Fatty Acids

Because tandem mass spectrometry- (MS/MS-) based newborn screening identifies many suspicious cases of fatty acid oxidation and carnitine cycle disorders, a simple, noninvasive test is required to confirm the diagnosis. We have developed a novel method to evaluate the metabolic defects in peripheral blood mononuclear cells loaded with deuterium-labeled fatty acids directly using the ratios of acylcarnitines determined by flow injection MS/MS. We have identified diagnostic indices for the disorders as follows: decreased ratios of d₂₇-C14-acylcarnitine/d₃₁-C16-acylcarnitine and d₂₃-C12-acylcarnitine/d₃₁-C16-acylcarnitine for carnitine palmitoyltransferase-II (CPT-II) deficiency, decreased ratios of d₂₃-C12-acylcarnitine/d₂₇-C14-acylcarnitine for very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, and increased ratios of d₂₉-C16-OH-acylcarnitine/d₃₁-C16-acylcarnitine for trifunctional protein (TFP) deficiency, together with increased ratios of d₇-C4-acylcarnitine/d₃₁-C16-acylcarnitine for carnitine palmitoyltransferase-I deficiency. The decreased ratios of d₁-acetylcarnitine/d₃₁-C16-acylcarnitine could be indicative of β-oxidation ability in patients with CPT-II, VLCAD, and TFP deficiencies. Overall, our data showed that the present method was valuable for establishing a rapid diagnosis of fatty acid oxidation disorders and carnitine cycle disorders and for complementing gene analysis because our diagnostic indices may overcome the weaknesses of conventional enzyme activity measurements using fibroblasts or mononuclear cells with assumedly uncertain viability.

Peroxisomes can oxidize medium- and long-chain fatty acids through a pathway involving ABCD3 and HSD17B4

Peroxisomes are essential organelles for the specialized oxidation of a wide variety of fatty acids, but they are also able to degrade fatty acids that are typically handled by mitochondria. Using a combination of pharmacological inhibition and clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated protein 9 genome editing technology to simultaneously manipulate peroxisomal and mitochondrial fatty acid β-oxidation (FAO) in HEK-293 cells, we identified essential players in the metabolic crosstalk between these organelles. Depletion of carnitine palmitoyltransferase (CPT)2 activity through pharmacological inhibition or knockout (KO) uncovered a significant residual peroxisomal oxidation of lauric and palmitic acid, leading to the production of peroxisomal acylcarnitine intermediates. Generation and analysis of additional single- and double-KO cell lines revealed that the D-bifunctional protein (HSD17B4) and the peroxisomal ABC transporter ABCD3 are essential in peroxisomal oxidation of lauric and palmitic acid. Our results indicate that peroxisomes not only accept acyl-CoAs but can also oxidize acylcarnitines in a similar biochemical pathway. By using an Hsd17b4 KO mouse model, we demonstrated that peroxisomes contribute to the plasma acylcarnitine profile after acute inhibition of CPT2, proving in vivo relevance of this pathway. We summarize that peroxisomal FAO is important when mitochondrial FAO is defective or overloaded.-Violante, S., Achetib, N., van Roermund, C. W. T., Hagen, J., Dodatko, T., Vaz, F. M., Waterham, H. R., Chen, H., Baes, M., Yu, C., Argmann, C. A., Houten, S. M. Peroxisomes can oxidize medium- and long-chain fatty acids through a pathway involving ABCD3 and HSD17B4.

Carnitine palmitoyltransferase II deficiency with a focus on newborn screening

Carnitine palmitoyltransferase (CPT) II deficiency is one of the most common forms of mitochondrial fatty acid oxidation disorder. Its clinical phenotypes are classified into the muscle, severe infantile, and lethal neonatal forms. Among Caucasians, the muscle form predominates, and the c.338C > T (p.S113L) variant is detected in most cases, whereas among the Japanese, c.1148T > A (p.F383Y) is the variant allele occurring with the highest frequency and can apparently cause symptoms of the severe infantile form. Newborn screening (NBS) for this potentially fatal disease has not been established. We encountered an infantile case of CPT II deficiency not detected in NBS using C16 and C18:1 concentrations as indices, and therefore we adopted the (C16 + C18:1)/C2 ratio as an alternative primary index. As a result, the disease was diagnosed in nine of 31 NBS-positive subjects. The values for (C16 + C18:1)/C2 in the affected newborns partly overlapped with those in unaffected ones. Among several other indices proposed previously, C14/C3 has emerged as a more promising index. Based on these findings, nationwide NBS for CPT II deficiency using both (C16 + C18:1)/C2 and C14/C3 as indices was officially approved and started in April 2018. We diagnosed the disease in four young children presenting with symptoms of the muscle form, whose values for the new indices were not elevated. Although it is still difficult to detect all cases of the muscle form of CPT II deficiency in NBS, our system is expected to save many affected children in Japan with the severe infantile form predominating.

Newborn screening for carnitine palmitoyltransferase II deficiency using (C16+C18:1)/C2: Evaluation of additional indices for adequate sensitivity and lower false-positivity

BACKGROUND

Carnitine palmitoyltransferase (CPT) II deficiency is one of the most common forms of mitochondrial fatty acid oxidation disorder (FAOD). However, newborn screening (NBS) for this potentially fatal disease has not been established partly because reliable indices are not available.

METHODS

We diagnosed CPT II deficiency in a 7-month-old boy presenting with hypoglycemic encephalopathy, which apparently had been missed in the NBS using C16 and C18:1 concentrations as indices. By referring to his acylcarnitine profile from the NBS, we adopted the (C16+C18:1)/C2 ratio (cutoff 0.62) and C16 concentration (cutoff 3.0nmol/mL) as alternative indices for CPT II deficiency such that an analysis of a dried blood specimen collected at postnatal day five retroactively yielded the correct diagnosis. Thereafter, positive cases were assessed by measuring (1) the fatty acid oxidation ability of intact lymphocytes and/or (2) CPT II activity in the lysates of lymphocytes. The diagnoses were then further confirmed by genetic analysis.

RESULTS

The disease was diagnosed in seven of 21 newborns suspected of having CPT II deficiency based on NBS. We also analyzed the false-negative patient and five symptomatic patients for comparison. Values for the NBS indices of the false-negative, symptomatic patient were lower than those of the seven affected newborns. Although it was difficult to differentiate the false-negative patient from heterozygous carriers and false-positive subjects, the fatty acid oxidation ability of the lymphocytes and CPT II activity clearly confirmed the diagnosis. Among several other indices proposed previously, C14/C3 completely differentiated the seven NBS-positive patients and the false-negative patient from the heterozygous carriers and the false-positive subjects. Genetic analysis revealed 16 kinds of variant alleles. The most prevalent, detected in ten alleles in nine patients from eight families, was c.1148T>A (p.F383Y), a finding in line with those of several previous reports on Japanese patients.

CONCLUSIONS

These findings suggested that CPT II deficiency can be screened by using (C16+C18:1)/C2 and C16 as indices. An appropriate cutoff level is required to achieve adequate sensitivity albeit at the cost of a considerable increase in the false-positive rate, which might be reduced by using additional indices such as C14/C3.